This invention relates to the field of liquid electrophotography, and specifically to a method and apparatus for screening liquid toners and receptors for use in electrophotographic printing devices.
In electrophotographic and electrostatic and imaging processes (collectively electrographic processes), an electrostatic image is formed on the surface of a photoreceptive element or dielectric element, respectively. The photoreceptive element or dielectric element may be an intermediate transfer sheet, drum or belt or the substrate for the final toned image itself, as described by Schmidt, S. P. and Larson, J. R. in Handbook of Imaging Materials, Diamond, A. S., Ed: Marcel Dekker: New York; Chapter 6, pp 227-252, and U.S. Pat. Nos. 4,728,983; 4,321,404; and 4,268,598.
In electrostatic printing, a latent image is typically formed by (1) placing a charge image onto a dielectric element (typically the receiving substrate) in selected areas of the element with an electrostatic writing stylus or its equivalent to form a latent charge image. This latent charge image is developed or toned by (2) applying toner to the charge image, and (3) fixing the toned image. An example of this type of process is described in U.S. Pat. No. 5,262,259.
In electrophotographic printing, also referred to as xerography, electrophotographic technology is used to produce images on a final image receptor, such as paper, film, drums, or the like. Electrophotographic technology is incorporated into a wide range of equipment including photocopiers, laser printers, facsimile machines, and the like.
Electrophotography typically involves the use of a reusable, light sensitive, temporary charge accepting, temporary image receptor, known as a photoreceptor. The photoreceptor is used in the process of producing an electrophotographic image on a final, permanent image receptor. A representative electrophotographic process involves a series of steps to produce a visible toned image on a receptor, including charging of the photoreceptor, exposure to dissipate the charge in an imagewise manner and form a latent charge image, toner development of the latent charge image, transfer of the toned image, fusing of the transferred toned image, cleaning of the photoreceptor, and erasure of residual charge on the photoreceptor.
In the charging step, a photoreceptor is covered with charge of a desired polarity, either negative or positive, typically with a corona device or charging roller. In the exposure step, an optical system, typically a laser scanner or diode array, forms a latent charge image by selectively discharging the charged surface of the photoreceptor in an imagewise manner corresponding to the desired image to be formed on the final image receptor. In the development step, toner particles of the appropriate polarity are generally brought into contact with the latent charge image on the photoreceptor, typically using a developer that is electrically-biased to a potential opposite in polarity to the toner polarity. The toner particles migrate to the photoreceptor and selectively adhere to the latent charge image via electrostatic forces, forming a temporary toned image on the photoreceptor.
In the transfer step, the temporary toned image is transferred from the photoreceptor to the desired final image receptor. An intermediate transfer element is sometimes used to effect transfer of the toned image (usually to accomplish a desired order of color planes in the image) from the photoreceptor with subsequent transfer of the toned image to a final image receptor. In the fusing step, the toned image on the final image receptor is heated to soften or melt the toner particles, thereby fusing the toned image to the final receptor to form a final and permanent image. An alternative fusing method involves fixing the toner to the final receptor under high pressure with or without heat. In the cleaning step, residual toner remaining on the photoreceptor is removed.
Finally, in the erasing step, the photoreceptor charge is reduced to a substantially uniformly low value by exposure to light of a particular wavelength band, thereby removing remnants of the original latent image and preparing the photoreceptor for the next imaging cycle.
Two types of toner are in widespread, commercial use: liquid toner and dry toner. The term xe2x80x9cdryxe2x80x9d does not mean that the dry toner is totally free of any liquid constituents, but connotes that the toner particles do not contain any significant amount of solvent (or gives the toner a liquid appearance), e.g., typically less than 10 weight percent solvent and preferably less then 8% or less then 5% by total weight of toner (generally, dry toner is as dry as is reasonably practical in terms of solvent content), and the dry toner particles are capable of carrying a triboelectric charge. This relative proportion of liquid carrier is a physical characteristic that distinguishes dry toner particles from liquid toner particles.
A typical liquid toner composition generally includes toner particles suspended or dispersed in a liquid carrier. The liquid carrier is typically a nonconductive dispersant liquid, the lack of charge carrying capability being necessary to avoid discharging the latent electrostatic image. Liquid toner particles are generally solvated or stabilized (dispersed and suspended) to some degree in the liquid carrier (or carrier liquid), typically in more than 50 weight percent (by total weight of the toner) of a low polarity, low dielectric constant, substantially nonaqueous carrier solvent. Liquid toner particles are generally chemically charged using polar groups that dissociate in the carrier solvent, but the toner particles do not carry a triboelectric charge while solvated and/or dispersed in the liquid carrier. Liquid toner particles are also typically smaller than dry toner particles. Because of their small particle size, ranging from about 5 microns to sub-micron size, liquid toners are capable of producing very high-resolution toned images.
A typical toner particle for a liquid toner composition generally comprises a visual enhancement additive (for example, a colored pigment particle) and a polymeric binder. The polymeric binder fulfills functions both during and after the electrophotographic process, supporting the visual enhancement additive during toning and fusing the visual enhancement additive during formation of the permanent image. With respect to processability, the character of the binder impacts charging and charge stability, flow, and fusing characteristics of the toner particles. These characteristics are important to achieve good performance during development, transfer, and fusing. After an image is formed on the final receptor, the nature of the binder (e.g., glass transition temperature, melt viscosity, molecular weight) and the fusing conditions (e.g., temperature, pressure and fuser configuration) impact the durability (e.g., blocking and erasure resistance), adhesion to the receptor, gloss, and the like.
Polymeric binder materials suitable for use in liquid toner particles typically exhibit glass transition temperatures of from about xe2x88x9224xc2x0 C. to 55xc2x0 C., which is lower than the range of glass transition temperatures (50-100xc2x0 C.) typical for polymeric binders used in dry toner particles. In particular, some liquid toners are known to incorporate polymeric binders exhibiting glass transition temperatures (Tg) below room temperature (25xc2x0 C.) to rapidly self fix, e.g., by film formation, in the liquid electrophotographic imaging process; see e.g., U.S. Pat. No. 6,255,363. However, such liquid toners arc also known to exhibit inferior image durability (e.g., poor blocking properties and erasure resistance) resulting from the low Tg after fusing the toned image to a final image receptor.
In other printing processes using liquid toners, self-fixing is not required. In such a system, the image developed on the photoconductive surface is transferred to an intermediate transfer belt (xe2x80x9cITBxe2x80x9d) or intermediate transfer member (xe2x80x9cITMxe2x80x9d) or directly to a print medium without film formation at this stage. See, for example, U.S. Pat. No. 5,410,392 to Landa, issued on Apr. 25, 1995; and U.S. Pat. No. 5,115,277 to Camis, issued on May 19, 1992. In such a system, this transfer of discrete toner particles in image form is carried out using a combination of mechanical forces, electrostatic forces, and thermal energy. In the system particularly described in the U.S. Pat. No. 5,115,277 Camis patent, DC bias voltage is connected to an inner sleeve member to develop electrostatic forces at the surface of the print medium for assisting in the efficient transfer of color images.
The toner particles used in such a system have been previously prepared using conventional polymeric binder materials, and not polymers made using an organosol process. Thus, for example the U.S. Pat. No. 5,410,392 Landa patent states that the liquid developer to be used in the disclosed system is described in U.S. Pat. No. 4,794,651 (also to Landa), issued on Dec. 27, 1988. This former Landa patent discloses liquid toners made by heating a preformed high Tg polymer resin in a carrier liquid to an elevated temperature sufficiently high for the carrier liquid to soften or plasticize the resin, adding a pigment, and exposing the resulting high temperature dispersion to a high energy mixing or milling process.
Although such non self-fixing liquid toners using higher Tg (Tg generally greater than or equal to about 60xc2x0 C.) polymeric binders should have good image durability, such toners are known to exhibit other problems related to the choice of polymeric binder, including image defects due to the inability of the liquid toner to rapidly self fix in the imaging process, poor charging and charge stability, poor stability with respect to agglomeration or aggregation in storage, poor sedimentation stability in storage, and the requirement that high fusing temperatures of about 200-250xc2x0 C. be used in order to soften or melt the toner particles and thereby adequately fuse the toner to the final image receptor.
To overcome the durability deficiencies, polymeric materials selected for use in both nonfilm-forming liquid toners and dry toners more typically exhibit a range of Tg of at least about 55-65xc2x0 C. to obtain good blocking resistance after fusing, yet typically require high fusing temperatures of about 200-250xc2x0 C. to soften or melt the toner particles and thereby adequately fuse the toner to the final image receptor. High fusing temperatures are a disadvantage for dry toners because of the long warm-up time and higher energy consumption associated with high temperature fusing and because of the risk of fire associated with fusing toner to paper at temperatures approximating or approaching the autoignition temperature of paper (233xc2x0 C).
In addition, some liquid and dry toners using high Tg polymeric binders are known to exhibit undesirable partial transfer (offset) of the toned image from the final image receptor to the fuser surface at temperatures above or below the optimal fusing temperature, requiring the use of low surface energy materials in the fuser surface or the application of fuser oils to prevent offset. Alternatively, various lubricants or waxes have been physically blended into the dry toner particles during fabrication to act as release or slip agents; however, because these waxes are not chemically bonded to the polymeric binder, they may adversely affect triboelectric charging of the toner particle or may migrate from the toner particle and contaminate the photoreceptor, an intermediate transfer element, the fuser element, or other surfaces critical to the electrophotographic process.
In addition to the polymeric binder and the visual enhancement additive, liquid toner compositions can optionally include other additives. For example, charge control agents can be added to impart an electrostatic charge on the toner particles. Dispersing agents can be added to provide colloidal stability, to aid fixing of the image, and to provide charged or charging sites for the particle surface. Dispersing agents are commonly added to liquid toner compositions because toner particle concentrations are high (inter-particle distances are small) and electrical double-layer effects alone will not adequately stabilize the dispersion with respect to aggregation or agglomeration. Release agents can also be used in the toner to help prevent the toner from sticking to fuser rolls when those are used. Other additives include antioxidants, ultraviolet stabilizers, antistatic agents, fungicides, bactericides, flow control agents, and the like.
One fabrication technique used in the manufacture of toners involves synthesizing an amphipathic copolymeric binder dispersed in a liquid carrier to form an organosol, then mixing the formed organosol with other ingredients to form a liquid toner composition. Typically, organosols are synthesized by nonaqueous dispersion polymerization of polymerizable compounds (e.g., monomers) to form copolymeric binder particles that are dispersed in a low dielectric hydrocarbon solvent (carrier liquid). These dispersed copolymer particles are sterically-stabilized with respect to aggregation by chemical bonding of a steric stabilizer (e.g., graft stabilizer), solvated by the carrier liquid, to the dispersed core particles as they are formed in the polymerization. Details of the mechanism of such steric stabilization are described in Napper, D. H., xe2x80x9cPolymeric Stabilization of Colloidal Dispersions,xe2x80x9d Academic Press, New York, N.Y., 1983. Procedures for synthesizing self-stable organosols are described in xe2x80x9cDispersion Polymerization in Organic Media,xe2x80x9d K. E. J. Barrett, ed., John Wiley: New York, N.Y., 1975.
Liquid toner compositions have been manufactured using dispersion polymerization in low polarity, low dielectric constant carrier solvents for use in making relatively low glass transition temperature (Tgxe2x89xa630xc2x0 C.) film-forming liquid toners that undergo rapid self-fixing in the electrophotographic imaging process. See, for example, U.S. Pat. Nos. 5,886,067 and 6,103,781. Organosols have also been prepared for use in making intermediate glass transition temperature (Tg between 30-55xc2x0 C.) liquid electrostatic toners for use in electrostatic stylus printers. See, for example, U.S. Pat. No. 6,255,363 B1. A representative non-aqueous dispersion polymerization method for forming an organosol is a free radical polymerization carried out when one or more ethylenically-unsaturated monomers, soluble in a hydrocarbon medium, are polymerized in the presence of a preformed, polymerizable solution polymer (e.g. a graft stabilizer or xe2x80x9clivingxe2x80x9d polymer). See U.S. Pat. No. 6,255,363.
Once the organosol has been formed, one or more additives can be incorporated, as desired. For example, one or more visual enhancement additives and/or charge control agents can be incorporated. The composition can then subjected to one or more mixing processes, such as homogenization, microfluidization, ball-milling, attritor milling, high energy bead (sand) milling, basket milling or other techniques known in the art to reduce particle size in a dispersion. The mixing process acts to break down aggregated visual enhancement additive particles, when present, into primary particles (having a diameter in the range of about 0.05 to 1.0 microns) and may also partially shred the dispersed copolymeric binder into fragments that can associate with the surface of the visual enhancement additive.
According to this embodiment, the dispersed copolymer or fragments derived from the copolymer then associate with the visual enhancement additive, for example, by adsorbing to or adhering to the surface of the visual enhancement additive, thereby forming toner particles. The result is a sterically-stabilized, nonaqueous dispersion of toner particles having a size in the range of about 0.1 to 2.0 microns, with typical toner particle diameters in the range 0.1 to 0.5 microns. In some embodiments, one or more charge control agents can be added after mixing, if desired.
Several characteristics of liquid toner compositions are important to provide high quality images. Toner particle size and charge characteristics are especially important to form high quality images with good resolution. Further, rapid self-fixing of the toner particles is an important requirement for some liquid electrophotographic printing applications, e.g., to avoid printing defects (such as smearing or trailing-edge tailing) and incomplete transfer in high-speed printing. Another important consideration in formulating a liquid toner composition relates to the durability and archivability of the image on the final receptor. Erasure resistance, e.g., resistance to removal or damage of the toned image by abrasion, particularly by abrasion from natural or synthetic rubber erasers commonly used to remove extraneous pencil or pen markings, is a desirable characteristic of liquid toner particles.
Another important consideration in formulating a liquid toner is the tack of the image on the final receptor. It is desirable for the image on the final receptor to be essentially tack-free over a fairly wide range of temperatures. If the image has a residual tack, then the image can become embossed or picked off when placed in contact with another surface (also referred to as blocking). This is particularly a problem when printed sheets are placed in a stack. Resistance of the image on the final image receptor to damage by blocking to the receptor (or to other toned surfaces) is another desirable characteristic of liquid toner particles.
To address some of these concerns, a film laminate or protective layer may be placed over the surface of the image. This laminate often acts to increase the effective dot gain of the image, thereby interfering with the accuracy of the color rendition of a color composite. In addition, lamination of a protective layer over a final image surface adds both extra cost of materials and extra process steps to apply the protective layer, and may be unacceptable for certain printing applications (e.g., plain paper copying or printing).
Various methods have been used to address the drawbacks caused by lamination. For example, approaches have employed radiation or catalytic curing methods to cure or crosslink the liquid toner after the development step in order to eliminate tack. Such curing processes are generally too slow for use in high speed printing processes. In addition, such curing methods can add significantly to the expense of the printing process. The curable liquid toners frequently exhibit poor self stability and crosslinking can result in brittleness of the printed ink.
Another method to improve the durability of liquid toned images and address the drawbacks of lamination is described in U.S. Pat. No. 6,103,781. This Patent describes a liquid ink composition containing organosols having side-chain or main-chain of crystallizable polymeric moieties. At column 6, lines 53-60, the authors describe a binder resin that is an amphipathic copolymer dispersed in a liquid carrier (also known as an organosol) that includes a high molecular weight (co)polymeric steric stabilizer covalently bonded to an insoluble, thermoplastic (co)polymeric core. The steric stabilizer includes a crystallizable polymeric moiety that is capable of independently and reversibly crystallizing at or above room temperature (22xc2x0 C.). According to the authors, superior stability of the dispersed toner particles with respect to aggregation is obtained when at least one of the polymers or copolymers (denoted as the stabilizer) is an amphipathic substance containing at least one oligomeric or polymeric component having a weight-average molecular weight of at least 5,000 which is solvated by the liquid carrier. In other words, the selected stabilizer, if present as an independent molecule, would have some finite solubility in the liquid carrier. Generally, this requirement is met if the absolute difference in Hildebrand solubility parameters between the steric stabilizer and the solvent is less than or equal to 3.0 MPa1/2.
As described in U.S. Pat. No. 6,103,781, the composition of the insoluble resin core is preferentially manipulated such that the organosol exhibits an effective glass transition temperature (Tg) of less than 22xc2x0 C., more preferably less than 6xc2x0 C. Controlling the glass transition temperature allows one to formulate an ink composition containing the resin as a major component so that the ink will undergo rapid film formation (rapid self-fixing) in liquid electrophotographic printing or imaging processes using offset transfer processes carried out at temperatures greater than the core Tg, preferably at or above 22xc2x0 C. (Column 10, lines 36-46). The presence of the crystallizable polymeric moiety that is capable of independently and reversibly crystallizing at or above room temperature (22xc2x0 C.) acts to protect the soft, tacky, low Tg insoluble resin core after fusing to the final image receptor. This acts to improve the blocking problem and erasure resistance of the fused, toned image at temperatures up to the crystallization temperature (melting point) of the crystallizable polymeric moiety.
In attempting to address tack of the image on a final receptor, one must also consider film strength and image integrity. As described in U.S. Pat. No. 6,103,781, for liquid electrophotographic toners (particularly liquid toners developed for use in offset transfer processes), the composition of the insoluble resin core is preferentially manipulated such that the organosol exhibits an effective glass transition temperature (Tg) of less than 22xc2x0 C., and more preferably less than 6xc2x0 C. Controlling the glass transition temperature allows one to formulate an ink composition containing the resin as a major component so that it will undergo rapid film formation (rapid self-fixing) in printing or imaging processes carried out at temperatures at least the core Tg, preferably at or above 22xc2x0 C. (Column 10, lines 36-46).
As can be seen from the preceding, liquid toners are inherently more complex than dry toners to formulate. After each iteration or formulation, the toners must be tested, or screened, to see how the changes affect actual printing and how well the changed toner will work in an actual printing device. When an electrophotographic system uses dry toner, the measurements of various toner properties can be taken (with multiple testers) and a direct correlation can be inferred to indicate if the toner will perform satisfactorily or not. In liquid electrophotography, the number and interrelationship of the variables is extremely complex. As a result, the current liquid toner screening processes require labor-intensive and time-intensive printing of each liquid toner to be tested on a prototype printing device to determine whether or not a toner will be satisfactory.
A simple screening technique is needed for matching the liquid toner with suitable receptors or vice verse to ensure the print quality from the toner. Furthermore, a liquid toner tends to age and changes its printing performance. A rapid, simple screening technique is needed for quality control to determine if a papery-looking image is due to toner change or receptor property change.
A liquid toner that transfers satisfactorily through a printing device may still fail upon final transfer to the final image receptor (which may be paper, overhead projection film, etc.). As a result, the current liquid toner screening processes require labor-intensive and time-intensive printing of each liquid toner to be tested on a prototype printing device using different final receptors to determine whether or not a toner will be satisfactory. Testing in this way is very inefficient and time-consuming because for each test, the toner to be tested must be poured into the toner cartridge for use. Each test is only minutes long, but once the test is complete, the cartridge must be disassembled and thoroughly cleaned before it can be reassembled and filled with the new toner.
The inclusion of final receptors of various thicknesses and textures may also be difficult because prototype machines are typically not designed to handle a wide variety of materials. Therefore, in order to test various receptors, the prototype machine must frequently be physically modified or rebuilt to accommodate, again resulting in time lost.
A testing procedure and method is provided for assessing the quality or acceptability of performance of individual liquid ink electrostatic toners on individual receptor surfaces. Rather then performing an actual run of an electrostatic imaging device with the individual toner and the individual receptor, a separate apparatus and method that does not use electrostatic imaging is used to test the interrelated properties of the toner and the receptor under reproducible conditions. An approximately standard drop of the individual liquid toner is placed on the receptor to be tested. The drop is pressed onto the receptor and spread on the surface of the receptor (preferably before the drop has had time to partially evaporate or to have the liquid in the drop absorb or naturally spread on the receptor surface) under controlled conditions. Various characteristics of the spread drop on the receptor surface are measured, and the characteristics are compared to parameters defining the characteristics expected from a liquid toner that define acceptable performance between toner and receptor. In this manner the relative performance of individual toners on individual receptor surfaces can be evaluated independent of electrostatic effects. This independent evaluation can be important as the electrostatic effects bring another parameter of performance into evaluation of the compatibility of the toner and receptor and can misdirect research for adjusting their compatibility.